Known dental milling tools or dental milling cutters have a ball end portion with curved portions of blades or cutting edges and an adjoining axial cutting portion with helical portions of blades or cutting edges and are, in their diameter, adjusted to tooth replacement parts or dental prostheses to be worked out of pre-sintered ceramic blanks or white bodies, especially zirconium oxide white bodies. That is, the diameter of the ball end, with which the dental implant or the like is to be generated, is chosen such that the tooth geometry, being provided with three-dimensional curved surfaces and notches, and not following a simple basic form, can be created with the required surface smoothness. However, for reasons of strength, the diameter can not be selected too small. A dental nose end mill intended for the processing of unfired high performance ceramics such as zirconium oxide and alumina oxide, can be found for example in the catalog 2009, S. 70 of the company Datron AG under the name “DATRON-VHM-Zirkonoxid-Dentalfräser”.
For the preparation of dental prostheses such as dental implants, ceramics are employed, which are able to be sintered, due to good hygienic properties and strength values, nowadays zirconia all-ceramic materials. Therein, an unsintered ceramic blank, a so-called green body, is pre-sintered to a certain rigid hardness, ie to a so-called white body (or pre-sintered body), on which a processing with dental burs or milling tools is still easily possible, but the shrinkage occurring during sintering up to the hardness of the white body is completed before the final shaping. A dental implant white body is then sintered out, which has to be fully-sintered or sintered through, but which is already in the shape of the final dental implant. In addition to dental implants, in the same way, bridges and other dental prostheses or dental restorations are made, in particular the supporting frameworks for crowns composed of zirconium oxide, more specifically zirconia all-ceramics, which contain, in addition to the present polycrystalline zirconium oxides further stabilizing oxides such as yttrium or magnesium, e.g. 3Y-TZP, YSZ or TZ-3Y. After shaping by freeform milling on the pre-sintered white body, the resulting pre-sintered prosthesis preparation or the resulting dental prosthesis white body is then finish sintered or fully sintered, wherein the occurring sintering shrinkage or the shrinkage in volume (often approx. 50%) must be considered in the pre-shapening milling process, as the dense sintered material can not be machined or only within narrow limits, without that the structure of the ceramic takes damage.
For milling machining of the dental prosthesis white bodies, manual processing methods such as manual copy milling are known.
Here, a plastic or plaster model of the dentition is made first in the dental laboratory, e.g. from a dental impression taken by a dentist. To disconnect excessive material during the completion of such models consisting of relatively soft, but often adhering material, hand-held pneumatic turbine grinders are available to the dental technician. In most cases milling tools with bud-shaped, often with staggered toothing, right-hand or left-hand twisted or spinned (with right-hand or left-hand helix) and relatively large grinding heads in the order of a tooth are used, having broad and deep gashes to prevent clogging. The grinding head is brazed to a substantially thinner shaft or shank, so that it can be cut over the entire circumference. Such a tool can be seen for instance in the product information “Hartmetallfräser SGFA, 2007” of the company Brasseler GmbH & Co. KG.
After that, the model can then be scanned and in parallel to that, the relevant dental prosthesis white body can be milled out of the pre-sintered zirconium oxide round or plate-shaped blank. Therein, dental milling tool and scanners are clamped in parallel to one another on a corresponding copy milling machine, e.g. Titian Mill of Schütz GmbH, wherein undercuts on the prosthesis white body can be produced by a pivoting of the worktop, but, for rough and fine processing and manual rework, tool changes and various fixtures of the worktop are needed.
Also in the dental field, more and more CNC milling prevails, wherein travel paths are generated using CAD/CAM-data, on which can be traveled along on multi-axis CNC milling machines by the machine in three dimensions, wherein modern CNC milling machines in addition to the three motion axes usually also have two further pivot axes, so that undercuts can be formed. The CAD/CAM-data is extracted from the scanned model or the denture scanned for instance by the dentist, so that one can speak of a computerized copy milling here, wherein the model building can be saved and also in milling even less is to do manually.
For instance, it is known from German Patent DE 696 25 012 T2, to cut out dental molds by means of CAD/CAM-generated travel paths from a suitable substrate and to insert between pairs of these moldings a polymerizable acrylate, in order to form an artificial tooth with different layers, such as a dental enamel layer, a shade layer and a backing layer. As a substrate for the forms, e.g. ceramic is proposed. The artificial tooth itself is not milled out of the substrate, but formed between tooth moldings of polymerizable acrylate.
Other automated 3D-shape milling processes or free form milling processes serve for the direct manufacturing of dental ceramics (white sintered) by milling of the dental prosthesis from pre-sintered ceramic white bodies, wherein, subsequently, the dental prosthesis is through-sintered or fully-sintered. Such a milling method is disclosed in WO2004/086999A1, for instance.
For this purpose, usually end mills having a hemispherical rounded ball end and one to four right twisted flutes are used, which have correspondingly one to four cutting edges on the outer edges of the cutting edges arranged between the flutes. For the elaboration of the dental prosthesis white body, the cutter is conveniently set from above onto the solid or full material of the respective pre-sintered ceramic round blank, and then it is step by step proceeded into the solid.
However, flaking, spalling or ruptures relatively often occurred on the partially sintered and thus relatively brittle ceramic. For the dental technician, these ruptures on the white body are at the same time the change criterion for the tool, because it can not be determined with certainty whether the tool has become dull or flaking results from the force applied on the ceramic by the milling.
Although from the processing of relatively soft materials such as plastic, wood, or as above mentioned, of gypsum, also left-hand twisted milling tools are known, which would have the advantage that during milling, no tensile force would act on the workpiece, wherein in the machining of ceramics in particular tensile forces cause flaking, as ceramics from the above mentioned type have a relatively low tensile strength even in the white-sintered or pre-sintered state. An example of a left twisted milling tool for processing plastic, aluminum, brass or copper is provided, for example, in the 2009 catalog, page 14 of the company Datron AG under the name “DATRON VHM-Einschneider, Linksspiral rechtsschneidend”. The milling tool is embodied as a single flute or single-edged tool in order for providing the large width and depth of the flute, which are usual for the machining of these materials. However, such tools can be used only if the chip discharge in downwards direction is possible. That is, not in processing cases such as 3D free-form milling, in which the milling tool is set onto the top of the solid material, but only in processing cases, in which a workpiece is machined on its vertical outer sides and the chips can be discharged downwards. Indeed, by the left-hand twist of the flutes, machining is advantageously carried out without tensile force. Thereby, the chips would be pressed downwards and would therefore lead to clogging of the tool, if chip discharge or removal is not possible in the downwards direction.